Why developers use pure POS instead of POW or some mixed types of proof of importance?
It's very simple, POS requires a lot less processing power than POW and other analogs of proofs of importance. Another important feature is that the number of hacker attacks will be significantly less because the attacker will not be able to screw up without having more than 51% of the total pool of coins .
What is the minimum master-node reward on this platform?
It all depends on how many people initially invested in tokens.
If I wanted to become a master-node, I need to buy tokens, then download the wallet and set up master-node?
Yes, that's right, there is detailed information on the official website of the platform, there is a detailed instruction on how to properly install the wallet and much more
Would you invest in such a project yourself? I'm just curious to hear your opinion.